The present invention is generally directed to a module and, more specifically, an electronic module.
Electronic modules have been widely utilized in the automotive industry and may take various forms, such as an all silicon ignition (ASI) module implemented in a TO247 package, as is shown in FIG. 1.
FIG. 1 depicts an exemplary electronic module 100 constructed according to the prior art. The electronic module 100 includes an electrically conductive tab/header or base plate 102 that may act as a ground plane and be connected to one or more of a plurality of conductive lead pins 104. A silicon die 106 that includes circuitry to implement a transistor, such as an insulated gate bipolar transistor (IGBT), may be configured such that a drain of the transistor is brought out on a face of the die 106 coupled to the base plate 102. In this configuration, a gate and source of the transistor are brought out on a face of the die 106 opposite the drain. A substrate 108, such as an alumina substrate, may provide interconnecting paths for a plurality of electronic components, such as a chip capacitor 112 and an application specific integrated circuit (ASIC) 110, and may also provide bond pads 114 for coupling the various associated components of the substrate 108 to one or more of the lead pins 104 and/or to circuitry integrated within the die 106. In a typical such assembly, the electronic components are encased in an epoxy mold compound 116. The epoxy mold compound may serve to seal the electronic components from the environment and may also be utilized to better match a coefficient of thermal expansion (CTE) of the various components located within the assembly 100.
In spite of the standardized manufacturing processes utilized to assemble the electronic module 100, various components, such as a solder attachment point, wire bonds and the molding compound, have exhibited negative characteristics, e.g., solder joint cracking, wire bond failure and delamination, respectively. It should be appreciated that these failures may seriously compromise the robustness and long-term reliability of the module 100. At the very least, such failure causes severe performance degradation and may also result in complete field failures of the module 100.
With reference to FIG. 2, delamination areas 120A and 120B indicate where delamination failures have frequently occurred in the module 100. In general, the cause of the delamination failures in the areas 120A and 120B are the result of CTE mismatches between the various layers of the module 100. For example, a typical epoxy molding compound has a coefficient of thermal expansion between about 10 ppm/xc2x0 C. and 13 ppm/xc2x0 C., with copper having a CTE of approximately 17 ppm/xc2x0 C. and a typical silicon die having a CTE of approximately 3 ppm/xc2x0 C. It should be appreciated that the mismatch in the CTE between the various components induces thermal stress at the interface of the various layers of the module 100, when the module 100 is subjected to temperature excursions from its stress-free molding temperature.
The interface between the molding compound 116 and a top surface of the header 102 is subject to both tensile and shearing stresses, which can result in delamination at this interface when the resultant state of stress of the interface exceeds the adhesion strength of the molding compound 116 to the copper of the header 102.
With reference to FIG. 3, a diagram 300 shows the regions of the interface that are in tension (tension areas 304) and compression (compression areas 302). From various simulations, it is apparent that the likelihood of delamination is the greatest in the tension areas 304. It should also be appreciated that the tendency to delaminate is further exacerbated when the maximum shear stress exceeds the interfacial adhesion strength in shear. When the header 102 is plated with nickel, which has an extremely low adhesion strength (approximately 0 MPa) to a typical molding compound, the delamination problem is further exacerbated. Inherently low adhesion strength in the presence of shearing and tensile stresses at the interface between the header 102 and the molding compound 116 increases the likelihood that the molding compound 116 will delaminate from the header 102 and, in fact, it has been a persistent problem with the design of the module 100.
Delamination of the molding compound 116 from the header 102 negates one of the primary purposes for which the molding compound 116 is used in the module 100. That is, the molding compound 116 has typically been selected such that its CTE is in between the CTE of the silicon die 106 and the header 102 to provide a buffer layer.
Utilizing a molding compound 116 that has a CTE value between that of the silicon die 106 and the header 102 generally causes the silicon die 106 to contract more than it would if it was free-standing and causes the header 102 to contract less than it would if it was free-standing. In other words, the CTE mismatch between the silicon die 106 and the header 102 is reduced by the molding compound 116. Reduction of the CTE mismatch between the silicon die 106 and the header 102 results in solder joints between the two devices, experiencing a lower shear stress and, as such, showing an improved resistance to fatigue-induced cracking. However, when the molding compound 116 delaminates from either the silicon die 106 or the header 102 (or both), it no longer provides an effective buffer. At least one approach to addressing this problem has been to apply a coating of a polyimide material onto the header 102 and the other components mounted to the header 102 prior to encapsulation with the molding compound 116. In such an embodiment, the polyimide material acts as an adhesion promoter and increases the adhesion strength of the molding compound 116 to the header 102. However, the addition of polyimide to the design generally results in increased cost and, at times, can be difficult to apply in a consistent manner such that coverages of the areas where no delamination is acceptable are adequately covered.
What is needed is an electronic module that exhibits reduced delamination problems.
One embodiment of the present invention is directed to an electronic module that includes a plurality of electrically conductive lead pins, an electrically conductive base plate, a first integrated circuit (IC) die, at least one material block and an electrically non-conductive overmold. The first IC die is attached to the base plate and is electrically coupled to one or more of the lead pins. The at least one material block is positioned adjacent the die and attached to the base plate. The material block has a coefficient of thermal expansion (CTE) that is greater than the CTE of the die and less than the CTE of the base plate. The electrically non-conductive overmold encapsulates the die, the material block, the base plate and a portion of the lead pins.
According to another embodiment of the present invention, the overmold is an epoxy molding compound. According to still other embodiments of the present invention, the base plate is made of a nickel-plated copper and the material block is made of alumina. In at least one embodiment, the material block is rectangular and has about the same thickness as the die. In another embodiment, the material block is attached to the base plate with solder. In yet another embodiment, the material block has a CTE of about 7 parts per million per degrees Celsius (ppm/xc2x0 C.), the base plate has a CTE of about 17 ppm/xc2x0 C. and the die has a CTE of about 3 ppm/xc2x0 C., with the overmold having a CTE in the range of 10 ppm/xc2x0 C. to about 13 ppm/xc2x0 C.
In still another embodiment, the module includes a substrate having a plurality of conductive traces and a plurality of electronic components electrically coupled to the substrate. The substrate is attached to the base plate and the electronic components include at least a second IC die that is electrically coupled to at least a portion of the conductive traces of the substrate and the first IC die.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.